bootstat: test: refine reported test durations

Use actual test durations to refine future duration estimates.
Better estimates are cosmetic, but lend confidence to the test
results.

SideEffects: none
Test: system/core/bootstat/boot_reason_test.sh
Bug: 63736262
Change-Id: I49143b78a6dc6fb21838a3d6c70b7eb5a8b4cba5